Adjust GNOME-Shell so that it uses a Guix theme by default.

Patch by Ludovic Courtès <ludo@gnu.org>.

diff -ur a/data/gnome-shell-theme.gresource.xml b/data/gnome-shell-theme.gresource.xml
--- a/data/gnome-shell-theme.gresource.xml	2020-04-21 01:47:07.910448605 -0400
+++ b/data/gnome-shell-theme.gresource.xml	2020-04-21 01:47:42.120963071 -0400
@@ -12,6 +12,8 @@
     <file alias="icons/message-indicator-symbolic.svg">message-indicator-symbolic.svg</file>
     <file>no-events.svg</file>
     <file>no-notifications.svg</file>
+    <file>guix-background.png</file>
+    <file>guix-logo.png</file>
     <file>pad-osd.css</file>
     <file alias="icons/eye-open-negative-filled-symbolic.svg">eye-open-negative-filled-symbolic.svg</file>
     <file alias="icons/eye-not-looking-symbolic.svg">eye-not-looking-symbolic.svg</file>
diff -ur a/data/theme/gnome-shell-sass/widgets/_login-dialog.scss b/data/theme/gnome-shell-sass/widgets/_login-dialog.scss
--- a/data/theme/gnome-shell-sass/widgets/_login-dialog.scss	2020-03-30 18:29:41.197531200 -0400
+++ b/data/theme/gnome-shell-sass/widgets/_login-dialog.scss	2020-04-21 01:51:12.214121888 -0400
@@ -88,7 +88,16 @@
   }
 }
 
-.login-dialog-logo-bin { padding: 24px 0px; }
+.login-dialog-logo-bin {
+    /* Make it a bit narrower than .login-dialog-user-list.  */
+    width: 12em;
+    height: 8em;
+    background: url(resource:///org/gnome/shell/theme/guix-logo.png);
+    background-color: transparent;
+    background-repeat: no-repeat;
+    background-size: contain;
+    background-position: center; }
+
 .login-dialog-banner { color: darken($osd_fg_color,10%); }
 .login-dialog-button-box { width: 23em; spacing: 5px; }
 .login-dialog-message { text-align: center; }
diff -ur a/data/theme/gnome-shell-sass/widgets/_screen-shield.scss b/data/theme/gnome-shell-sass/widgets/_screen-shield.scss
--- a/data/theme/gnome-shell-sass/widgets/_screen-shield.scss	2020-03-30 18:29:41.198531200 -0400
+++ b/data/theme/gnome-shell-sass/widgets/_screen-shield.scss	2020-04-21 01:52:22.645180571 -0400
@@ -66,8 +66,10 @@
 }
 
 #lockDialogGroup {
-  background-color: lighten(#2e3436, 8%);
-}
+  background-color: lighten(#2e3436, 8%) url(resource:///org/gnome/shell/theme/guix-background.png);
+  background-repeat: no-repeat;
+  background-size: cover;
+  background-position: center; }
 
 #unlockDialogNotifications {
   StButton#vhandle, StButton#hhandle {
